Textures are low res which is the biggest deal as are particles which are mostly a single colored pixel. Additionally, meshes are low on polycounts, items reuse meshes and textures as often as possible, and instead of having large pregenerated maps, it's done through random generation. There are also very few visual ui elements, and those that do exist are typically simple, low res, or otherwise reused such as the workbench using a common ui shared with the forge, stonecutter, etc.

Basically, very efficient use of space and assets.